1. Field Of The Invention
The present invention broadly relates to ceramic composites having an encasement member for maintaining the composite under compressive stress, and to methods of making the same. In particular, the invention relates to ceramic composites comprising a polycrystalline ceramic matrix embedding a filler and having an encasement member for maintaining the ceramic composite under compressive stress, and to methods of making the composites.
2. Description of Commonly Owned Patent Applications
The subject matter of this application is related to that of Commonly Owned U.S. patent applications Ser. No. 819,397, filed Jan. 17, 1986, as a continuation-in-part of Ser. No. 697,876, now abandoned filed Feb. 4, 1985, both in the names of Marc S. Newkirk et al and entitled "Composite Ceramic Articles and Methods of Making Same." This copending application discloses a novel method for producing a self-supporting ceramic composite by growing an oxidation reaction product from a parent metal into a permeable mass of filler.
The method of producing a self-supporting ceramic body by oxidation of a parent metal precursor is disclosed generically in Commonly Owned U.S. Pat. No. 4,713,360 which issued on Dec. 15, 1987 and was based on U.S. patent application Ser. No. 818,943, now U.S. Pat. No. 4,713,360, filed Jan. 15, 1986, as a continuation-in-part of Ser. No. 776,964, filed Sept. 17, 1985, as a continutation-in-part of Ser. No. 705,787, now abandoned, filed Feb. 26, 1985, as a continuation-in-part of Ser. No. 591,392, now abandoned filed Mar. 16, 1984, all in the name of Marc S. Newkirk et al and entitled "Novel Ceramic Materials and Methods of Making The Same." This discovery of an oxidation phenomenon, which may be enhanced by the use of a dopant alloyed in the parent metal, affords self-supporting ceramic bodies of desired size grown as the oxidation reaction product of the precursor parent metal.
The foregoing method was improved upon by the use of external dopants applied to the surface of the precursor parent metal as disclosed in Commonly Owned U.S. patent application Ser. Nos. 822,999,filed Jan. 27, 1986, as a continuation-in-part of Ser. No. 776,965, filed Sept. 17, 1985, as a continuation-in-part of Ser. No. 747,788, filed June 25, 1985, as a continuation-in-part of Ser. No. 632,636, filed July 20, 1984, all in the names of Marc S. Newkirk et al and entitled "Methods of Making Self-Supporting Ceramic Materials."
A further development of the foregoing methods enables the formation of self-supporting ceramic structures which contain therein one or more cavities which inversely replicate the geometry of a positive mold of shaped precursor parent metal embedded within a bed of conformable filler which is self-bonding under specified conditions as described in Commonly Owned U.S. patent application Ser. No. 823,542, filed Jan. 27, 1986, in the name of Marc S. Newkirk et al, entitled "Inverse Shape Replication Method of Making Ceramic Composite Articles and Articles Obtained Thereby."
Yet another development of the foregoing methods enables the formation of self-supporting ceramic bodies having a negative pattern which inversely replicates the positive pattern of a parent metal precursor emplaced against a mass of filler, as described in Commonly Owned U.S. patent application Ser. No. 896,157, filed Aug. 13, 1986, in the name of Marc S. Newkirk and entitled "Method of Making Ceramic Composite Articles With Shape Replicated Surfaces and Articles Obtained Thereby."
The entire disclosures of all of the foregoing Commonly Owned Patent Applications and those described below are expressly incorporated herein by reference.
3. Background and Prior Art
In recent years, there has been increasing interest in the use of ceramics for structural applications historically served by metals. The impetus for this interest has been the superiority of ceramics with respect to certain properties, such as corrosion resistance, hardness, modulus of elasticity, and refractory capabilities when compared with metals.
Current efforts at producing higher strength, more reliable, and tougher ceramic articles are largely focused upon (1) the development of improved processing methods for monolithic ceramics and (2) the development of new material compositions, notably ceramic matrix composites. A composite structure is one which comprises a heterogeneous material, body or article made of two or more different materials which are intimately combined in order to attain desired properties of the composite. For example, two different materials may be intimately combined by embedding one in a matrix of the other. A ceramic matrix composite structure typically comprises a ceramic matrix which encloses one or more diverse kinds of filler materials such as particulates, fibers, rods or the like.
The Commonly Owned Patent and Patent Applications describe new processes which resolve some of the problems or limitations of traditional ceramic technology for making composites such as by compacting and sintering. The present invention combines the processes of the Commonly Owned Patent and Patent Applications with additional novel concepts to provide for forming shaped ceramic composite structures having an encasement member formed integrally with the composite for maintaining it under compression. Further, the present invention provides for fabrication of ceramic composites of certain predetermined geometry by oxidation reaction phenomenon which overcomes the difficulties and limitations associated with known processes and further provides for an integral means formed in situ during the oxidation reaction process in maintaining the ceramic composite under compressive stress. It is known in the ceramic field that ceramics are generally much stronger in compressive strength than in tensile strength. If the ceramic is put under pressure, such as by applying pressure to the inside diameter of a ceramic tube, the ceramic is then put in tension. If a crack develops in the ceramic while under tension, the ceramic will shatter. Therefore, it is desirable to prevent this catastrophic failure, which can be accomplished by the method and product of this invention, by providing a compressive means formed in situ for maintaining the ceramic composite under compression.
It has long been known that favorably prestressed parts can appear very strong in application because stresses which such parts see in use may first actually serve to unload the prestress before effectively adding stress to the part. In the case of employing ceramics to tensile design applications, maximum design potential is achieved by prestressing such materials in compression, particularly since their compressive strengths may in some cases be a factor of 5 or more greater than their tensile strengths. Prior to the present invention, prestressing a ceramic part with a metal sleeve was a very costly and tricky business. The ceramic and metal component had to be a very close tolerance fit with near-perfect conformity between the two in order to generate the proper amount of uniform stress with respect to the ceramic and avoid the creation of very high contact stresses which could cause failure of the ceramic. Typically this condition has been only met by careful and expensive high-tolerance grinding of the ceramic part and metal sleeve at the surfaces where they interface. The present invention overcomes these problems and the need for such grinding by a novel technique as described herein.